In order to apply energy conservation to a falling ball or a roller coaster in the
previous chapter, we had to assume that friction (with the air or the track) was
negligible. Introduce some friction, and the falling ball’s kinetic energy doesn’t
grow quite as quickly, while the roller coaster coasts to a stop, even on a level track.
The effects of friction demonstrate that conservation of mechanical energy is not a
universal law of physics.
But besides draining objects of their mechanical energy, friction also has another
effect: It raises the temperature of objects. Everyone knows that you can warm up
cold hands by briskly rubbing them together. Tires skidding along the road result
in burnt rubber. A power drill’s bit can quickly become too hot to touch. And
although everyday falling objects don’t often become noticeably warmer, the space
shuttle requires state-of-the-art heat shields to prevent damage during its long, fast
descent into the atmosphere.